3693: a private music app in Flutter and Firebase
A private music app built with Flutter: search, a now-playing screen with lyrics, a queue, playlists, liked songs, a cloud library, listening stats and light and dark themes, on a typed Firebase backend.
Our role: Our own product, designed and built by App369 founder Simon Dziak, who made every commit in the repository. It is a private app for its owner and friends.
What 3693 Needed to Solve
The repository's own QA report calls 3693 "a small private app (owner plus friends)". That changes the brief. There is no store growth target, so the work goes into the parts a listener feels: audio that keeps playing in the background, a queue that stays correct when someone taps next five times in a second, and a library that follows the account from one device to the next. The commit history shows two phases: a burst of features in February, then correctness and polish in August. A 103-finding multi-agent QA review in between shaped the August hardening, and the lessons below come from it.
How We Built It
A Flutter app plays audio with just_audio and audio_service, so playback continues in the background and shows on the lock screen. GetX handles sign-in and routing, and a Provider-based player controller holds the queue and playback state. Hive keeps liked songs, recent searches, recently played tracks, playlists and settings on the device. Firebase Auth signs people in, and the cloud library and playlists live behind callable Cloud Functions written in TypeScript. Every function sits behind one auth wrapper that checks the caller before any code runs.
What the Product Does
Search and playback
Search with recent searches and trending chips, a mini player, and background playback with lock-screen controls and a working scrubber.
Now playing
A waveform seek bar, lyrics with the current line highlighted, shuffle, repeat, a sleep timer, a shader-based visualizer and haptic feedback.
Queue
A queue screen where tracks can be reordered or swiped away, and a retry that skips to the next track when one fails to load.
Library
Playlists, liked songs, recently played and a cloud library saved to the account.
Listening stats
A stats screen with top songs, top artists and an hour-of-day heatmap, plus a card that can be shared.
Light and dark themes
One palette object defines both themes, and the interface takes accent colors from the current artwork.
Accessibility
Screen-reader labels, reduced-motion handling (with a test) and a layout for tablets.
The Stack, Layer by Layer
- Client app
- Flutter and Dart. GetX for auth and routing, Provider for the audio controller, just_audio and audio_service for playback, Hive for local storage. The repository has about 18 screen files.
- Backend
- Cloud Functions (2nd gen) on Node.js 22 in TypeScript. Callable functions for account and profile, library and playlists, each with typed request and response interfaces and input validation.
- Auth middleware
- A requireAuth wrapper rejects unauthenticated calls and can also require a verified email or an admin flag. Handlers receive the verified user id rather than reading one from the request body.
- Security rules
- Firestore and Storage rules are kept in the same repository as the functions.
- Testing
- 52 Flutter test files cover controllers, services, screens and widgets. A separate device walkthrough signs in, visits every screen in both themes and saves a screenshot of each.
- Release
- A preflight script gates TestFlight archives (see the lessons below).
Verifiable Facts
Counted from the private source repository on October 13, 2026. This page makes no claims about listeners, installs or revenue.
What We Learned Shipping It
A test run can end up inside your release build
The device walkthrough runs with flutter drive, which rewrites the gitignored ios/Flutter/Generated.xcconfig so FLUTTER_TARGET points at the test file. An Xcode archive taken afterwards would ship the test as the app: it would sign in, flip themes and tap through tabs on its own. Nothing in the repository showed it. The fix is tool/release_preflight.sh, which regenerates the config from lib/main.dart, fails unless FLUTTER_TARGET is lib/main.dart, checks the analyzer and the tests, and prints the build number to bump.
Key things by id, never by title
The QA pass found song identity derived from the display title in many places: the now-playing highlight, the like button, the queue keys. Same-title songs collided. The worst case was listening stats, which were built on a box keyed by title, so a replay overwrote the earlier play and the counts were wrong. The report ranked it as the one critical finding.
Do not rebuild the tree on every position tick
The player was a single app-wide ChangeNotifier that notified listeners on each position update, about five times a second, so large parts of the interface rebuilt continuously while music played. The report also found animations and timers that never stopped when paused. The August commits worked through these in batches.
Have the polish reviewed by someone else
The QA pass was run by several AI agents across 12 areas, and every finding was re-checked against the code before it counted: 103 confirmed (1 critical, 7 high, 24 medium, 71 low). It also caught a build break, where the app's own RepeatMode enum collided with one newly exported by the Flutter SDK.
Replace a half-built feature with a real one
A high-contrast toggle saved its setting but no screen read it. The review caught it, and the August theme work replaced it with a single light/dark palette used by every screen.
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